1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to a method for performing random access in a wireless communication system.
2. Discussion of the Related Art
The E-UMTS system is an evolved version of the conventional WCDMA UMTS system and basic standardization thereof is in progress under the 3rd Generation Partnership Project (3GPP). The E-UMTS is also referred to as a Long Term Evolution (LTE) system. For details of the technical specifications of the UMTS and E-UMTS, refer to Release 7 and Release 8 of “3rd Generation Partnership Project; Technical Specification Group Radio Access Network”.
The E-UMTS mainly includes a User Equipment (UE), a base station (or eNB or eNode B), and an Access Gateway (AG) which is located at an end of a network (E-UTRAN) and which is connected to an external network. Generally, an eNB can simultaneously transmit multiple data streams for a broadcast service, a multicast service and/or a unicast service. The AG can be divided into a part that handles processing of user traffic and a part that handles control traffic. Here, the AG part for processing new user traffic and the AG part for processing control traffic can communicate with each other using a new interface. One or more cells may exist for one eNB. An interface for transmitting user traffic or control traffic can be used between eNBs. A Core Network (CN) may include the AG and a network node or the like for user registration of the UE. An interface for discriminating between the E-UTRAN and the CN can be used. The AG manages mobility of a UE on a Tracking Area (TA) basis. One TA includes a plurality of cells. When the UE has moved from a specific TA to another TA, the UE notifies the AG that the TA where the UE is located has been changed.
FIG. 1 illustrates a network structure of an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) system which is a mobile communication system to which the embodiment of the present invention is applied. The E-UTRAN system is an evolved version of the conventional UTRAN system. The E-UTRAN includes a base station that will also be referred to as “eNode B” or “eNB”. The eNBs are connected through an X2 interface. Each eNB is connected to the User Equipment (UE) through a radio interface and is connected to an Evolved Packet Core (EPC) through a S1 interface.
FIGS. 2 and 3 illustrate the configurations of a control plane and a user plane of a radio interface protocol between a UE and a UMTS Terrestrial Radio Access Network (UTRAN) based on the 3GPP radio access network standard. The radio interface protocol is divided horizontally into a physical layer, a data link layer and a network layer, and vertically into a user plane for data transmission and a control plane for signaling. The protocol layers of FIGS. 2 and 3 can be divided into an L1 layer (first layer), an L2 layer (second layer) and an L3 layer (third layer) based on the lower three layers of the Open System Interconnection (OSI) reference model widely known in communication systems.
The control plane is a passage through which control messages that a UE and a network use in order to manage calls are transmitted. The user plane is a passage through which data (e.g., voice data or Internet packet data) generated at an application layer is transmitted. The following is a detailed description of the layers of the control and user planes in a radio interface protocol.
The physical layer, which is the first layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to a Media Access Control (MAC) layer, located above the physical layer, through a transport channel. Data is transferred between the MAC layer and the physical layer through the transport channel. Data transfer between different physical layers, specifically between the respective physical layers of transmitting and receiving sides, is performed through the physical channel. The physical channel is modulated according to the Orthogonal Frequency Division Multiplexing (OFDM) method, using time and frequencies as radio resources.
The MAC layer of the second layer provides a service to a Radio Link Control (RLC) layer, located above the MAC layer, through a logical channel. The RLC layer of the second layer supports data transmission with reliability. The functions of the RLC layer may also be implemented through internal functional blocks of the MAC layer. In this case, the RLC layer need not be existed. A PDCP layer of the second layer performs a header compression function to reduce unnecessary control information in order to efficiently transmit IP packets such as IPv4 or IPv6 packets in a radio interface with a relatively narrow bandwidth.
A Radio Resource Control (RRC) layer located at the lowermost of the third layer is defined only in the control plane and is responsible for control of logical, transport, and physical channels in association with configuration, re-configuration and release of Radio Bearers (RBs). The RB is a service that the second layer provides for data communication between the UE and the UTRAN. To accomplish this, the RRC layer of the UE and the RRC layer of the network exchange RRC messages. The UE is in RRC connected mode if RRC connection has been established between the RRC layer of the radio network and the RRC layer of the UE. Otherwise, the UE is in RRC idle mode.
A Non-Access Stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.
One cell of the eNB is set to use a bandwidth such as 1.25, 2.5, 5, 10 or 20 MHz to provide a downlink or uplink transmission service to UEs. Here, different cells may be set to use different bandwidths.
Downlink transport channels for transmission of data from the network to the UE include a Broadcast Channel (BCH) for transmission of system information, a Paging Channel (PCH) for transmission of paging messages and a downlink Shared Channel (SCH) for transmission of user traffic or control messages. User traffic or control messages of a downlink multicast or broadcast service may be transmitted through a downlink SCH and may also be transmitted through a downlink multicast channel (MCH). Uplink transport channels for transmission of data from the UE to the network include a Random Access Channel (RACH) for transmission of initial control messages and an uplink SCH for transmission of user traffic or control messages.
Logical channels, which are located above the transport channels and are mapped to the transport channels, include a Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH), a Common Control Channel (CCCH), a Multicast Control Channel (MCCH) and a Multicast Traffic Channel (MTCH).
FIG. 4 is a view showing an example of a physical channel structure used in an E-UMTS system. A physical channel includes several subframes on a time axis and several subcarriers on a frequency axis. Here, one subframe includes a plurality of symbols on the time axis. One subframe includes a plurality of resource blocks and one resource block includes a plurality of symbols and a plurality of subcarriers. In addition, each subframe may use certain subcarriers of certain symbols (e.g., a first symbol) of a subframe for a physical downlink control channel (PDCCH), that is, an L1/L2 control channel. In FIG. 4, an L1/L2 control information transmission area (PDCCH) and a data area (PDSCH) are shown. In one embodiment, a radio frame of 10 ms is used and one radio frame includes 10 subframes. In addition, one subframe includes two consecutive slots. The length of one slot may be 0.5 ms. In addition, one subframe includes a plurality of OFDM symbols and a portion (e.g., a first symbol) of the plurality of OFDM symbols may be used for transmitting the L1/L2 control information. A transmission time interval (TTI) which is a unit time for transmitting data is 1 ms.
A base station and a UE mostly transmit/receive data via a PDSCH, which is a physical channel, using a DL-SCH which is a transmission channel, except a certain control signal or certain service data. Information indicating to which UE (one or a plurality of UEs) PDSCH data is transmitted and how the UE receive and decode PDSCH data is transmitted in a state of being included in the PDCCH.
For example, in one embodiment, a certain PDCCH is CRC-masked with a radio network temporary identity (RNTI) “A” and information about data is transmitted using a radio resource “B” (e.g., a frequency location) and transmission format information “C” (e.g., a transmission block size, modulation, coding information or the like) via a certain subframe. Then, one or more UEs located in a cell monitor the PDCCH using its RNTI information. And, a specific UE with RNTI “A” reads the PDCCH and then receive the PDSCH indicated by B and C in the PDCCH information.
FIG. 5 is a process flow diagram illustrating a contention-based random access procedure.
The random access procedure is used to transmit short-length data in uplink. For example, the random access procedure is performed upon initial access in an RRC idle mode, upon initial access after radio link failure, upon handover requiring the random access procedure, and upon the occurrence of uplink/downlink data requiring the random access procedure during an RRC connected mode. Some RRC messages such as an RRC connection request message, a cell update message, and an URA update message are transmitted using a random access procedure. Logical channels such as a Common Control Channel (CCCH), a Dedicated Control Channel (DCCH), or a Dedicated Traffic Channel (DTCH) can be mapped to a transport channel (RACH). The transport channel (RACH) can be mapped to a physical channel (e.g., Physical Random Access Channel (PRACH)). When a UE MAC layer instructs a UE physical layer to transmit a PRACH, the UE physical layer first selects an access slot and a signature and transmits a PRACH preamble in uplink. The random access procedure is divided into a contention-based procedure and a non-contention-based procedure.
As shown in FIG. 5, a UE receives and stores information regarding random access from an eNB through system information. Thereafter, when random access is needed, the UE transmits a random access preamble (message 1) to the eNB (S510). After transmitting the random access preamble (message 1), the UE monitors a PDCCH during a predetermined period of time in order to receive a random access response message. After receiving the random access preamble from the UE, the eNB transmits a random access response (message 2) to the UE (S520). Specifically, downlink scheduling information for the random access response message can be CRC-masked with a Random Access-RNTI and can be transmitted through an L1/L2 control channel (PDCCH). Upon receiving the downlink scheduling signal masked with the RA-RNTI, the UE can receive and decode a random access response message from a PDSCH. Thereafter, the UE checks whether or not a random access response corresponding to the UE is present in the received random access response message. Whether or not a random access response corresponding to the UE is present can be determined based on whether or not a RAID for the preamble that the UE has transmitted is present. After receiving response information, the UE transmits an uplink message (Message 3) through an uplink SCH according to information regarding radio resources included in the response information (S530). After receiving the uplink message from the UE, the eNB transmits a contention resolution message (Message 4) (S540).
When random access has failed, the UE performs back-off. Here, the term “back-off” refers to delaying, by a UE, an access attempt by an arbitrary or predetermined time. If the UE makes an access attempt immediately after random access has failed, the access attempt is likely to fail again for the same or similar reason. Accordingly, when random access has failed, the UE delays an access attempt by a predetermined time to prevent waste of radio resources due to failure of the access attempt and to increase the probability that the random access is successful.
FIG. 6 illustrates a method for signaling back-off information according to a conventional technology.
As shown in FIG. 6, an eNB transmits a back-off parameter to all UEs in the cell through system information (S610). Thereafter, the UE performs its own back-off setting using a back-off parameter obtained from the system information. When random access is needed, the UE transmits a preamble for random access to the eNB (S620). The preamble may include a Random Access IDentity (RAID). Thereafter, when a random access procedure has failed for some reason, the UE performs back-off (S630). Thereafter, the UE retransmits a preamble for random access to the eNB (S640).
In the conventional technology, the UE should receive and store a back-off parameter through system information before making random access since the back-off parameter was broadcast through system information. Since a back-off parameter should be periodically broadcast through system information, a downlink overhead is always broadcast even when back-off is not performed since random access is successful. In addition, it may also be necessary to apply a different back-off parameter due to a cause such as load. However, since a back-off parameter is broadcast through system information, each UE in the cell cannot perform different back-off.